1. Field of the Invention
The present invention relates generally to excavation tools for the mining industry, where fragmenting rock, coal and other minerals is accomplished by the use of drag bits. More particularly, the present invention relates to utilizing a double acting bit holder in lieu of known drag bit holders to improve the cutting action of the bit while providing a means for resharpening and maintaining the bit in a sharp condition during excavation use.
2. Description of the Prior Art
A large number of excavation machines used in the mining and construction industries are equipped with drag bits, and these machines include continuous miners, boring machines, road headers, saws, drills and trenchers. The cutting surfaces or heads of these machines may be equipped with a few to more than 100 individual drag bits, each of which is held in place by a bit holder or bit block. The types of bit holders will depend on the types of bits used, and the bits may be of a conical type, cutter type, or concave type. While there are different holders for different types of bits, each holder has a recess for the shank of the bit and some kind of mechanism to secure the bit to the holder.
The bit holders function to secure the replaceable drag bits securely to the cutting surface of the excavator, and the holders are simple devices with no moving parts that hold the bit secure in the proper cutting position. The cutting action of these drag cutters is a simple plowing motion that enables the bit to stay in contact with the material being cut. Nevertheless, the continuous plowing action produces a variety of small and large chips, and forms a zone of crushed material under the bit, and the materials under the crushed zone and the amount of small chips are indicative of inefficient and costly cutting that induces excessive bit wear and bit heating. The inefficient cutting motion is the first major disadvantage of conventional bit holders, and as the individual drag bits become worn out, they are replaced because there is no means for resharpening the bits during their cutting life. Therefore, as the bit becomes worn, the production rate falls off while the cutting forces, dust generation and damaging machine vibrations increase, and this process continues up to the end of the bits lives, where the bits may require up to five times the force and energy that would normally be required for new sharp bits.
Accordingly, the second major disadvantage of conventional bit holders is their inability to resharpen or keep the bit sharp over the course of its cutting life activity.
An exception to conventional bit holders with no moving parts are the pin mounted embodiments of U.S. Pat. No. 3,697,137 which allows rotation of the bit about a pin. However, these holders do not have a front sharpening position as defined by the front stop and thus cannot maintain or resharpen bits during cutting and they do not have the ability to provide an impact force to the bit in the direction of the cut to aid in the cutting process.